1. Field of the Invention
The present invention relates to an image heating apparatus suitably used as a fusing unit mounted on an image forming apparatus, such as an electrophotographic copying machine and a laser beam printer, and an image heating rotational body mounted on the image heating apparatus.
2. Description of the Related Art
As a fusing unit to be mounted on an electrophotographic image forming apparatus, a system is proposed (Japanese Patent Laid-Open No. 2003-186327) in that a heating member is arranged on the surface (outer circumferential surface) of a fusing roller and the fusing roller is heated from the outer circumferential surface side (referred to an externally heating system below). By heating only the outer circumferential surface of the fusing roller, a start-up period to a desired temperature as well as electric power consumption can be reduced. This externally heating type fusing unit is broadly classified into a contact type in that the heating member is arranged in contact with the surface of the fusing roller and a non-contact type in that the surface of the fusing roller is heated using a halogen heater as a heat source. The contact-type externally heating fusing unit has a merit of the high heat transfer efficiency in comparison with the non-contact type, because heat is transferred by directly bringing the heat source, such as a ceramic heater, into contact with the fusing roller.
However, in the contact-type externally heating fusing unit, the heating member is arranged in contact with the surface of the fusing roller, so that scratches may be generated on the surface of the fusing roller. If dust is pinched between the heating member and the fusing roller, the pinched foreign material slidably rubs the same position of the surface of the fusing roller, so that a scratch may be generated on the surface of the fusing roller along the rotational direction. During fusing toner images on a recording member, the surface configuration of the fusing roller is transferred onto the toner images on the recording member, image failure, such as a vertical streak, due to the scratch generated on the surface of the fusing roller may emerge on the fixed toner images.
The scratch on the surface of the fusing roller may be generated on not only the contact-type externally heating fusing unit but also on the fusing unit having the heat source inside the fusing roller. For example, when a number of the same-sized recording members are processed, a scratch may be generated in the boundary between a paper passage part and a non-paper passage part of the fusing roller. Such a scratch may also cause the image failure.
In the fusing roller having a releasing layer, such as a fluoreresin, as a surface layer, the surface of a new roller is a mirror plane and its surface roughness Rz is usually about 0.1 to 0.3 μm. Whereas, in the passage part of the recording member on the surface of the fusing roller, the surface is gradually devastated due to the damage from paper fibers and external additives, so that the scratch is gradually enlarged to the extent of a surface roughness Rz of 1.0 μm.
Since an edge part of paper is provided with burrs generated when cutting the paper, the edge part has a large effect on the fusing roller, so that the scratch is gradually enlarged to the extent of a surface roughness Rz of 1.0 to 2.0 μm. The paper burr is liable to be generated when the knife blade becomes blunt due to abrasion in the cutting process from large-sized paper.
In the non-paper passage part on the surface of the fusing roller, the recording member does not pass through; the surface layer of the fusing roller abuts a pressure member, which forms the nip together with the fusing roller; and the scratch is enlarged to the extent of a surface roughness Rz of 1.0 μm slowly compared with in the paper passage part.
As a result, the surface roughness of the fusing roller after continuous paper processing increases in the order of (3) the paper edge passage part>(1) the paper passage part>(2) the non-paper passage part>the initial state (new roller). Hence, as the use proceeds, the surface state of the fusing roller differs dependently on the position in the generating line direction.
Then, the surface state of the fusing roller and the glossiness unevenness on images will be described.
When fusing unfixed toner images, the fusing unit applies pressure and heat to the toner. At this time, the surface micro-configuration of the fusing roller is transferred onto the surface of fixed toner images. If the surface state of the fusing roller differs, the surface state of the toner images is differentiated along with this, resulting in glossiness unevenness. This phenomenon is significant in coated paper excellent in surface smoothness while being in an invisible level for office-use normal paper. According to the study by the inventor, the scratch generation due to paper edges depends on the paper kind; the scratch generation level is deteriorated for paper having burrs generated when cutting and the level is similar to this example for other thick paper and coated paper.
In general, the high glossiness is recognized when the reproducibility of specular reflected light images is high while the low glossiness is recognized when the reproducibility is low or none. For example, when viewing silver-film photographic images under fluorescent lightning, not only the fluorescent light is reflected, but also the shape of the fluorescent lamp is transferred, so that the high glossiness is recognized independently of consciousness. This indicates that the photographic images are in a mirror plane state with small unevenness.
On the other hand, in the case of low glossiness, the surface state of images is reversely uneven, so that the fluorescent light is diffusely reflected and the shape of the fluorescent lamp is not transferred. In such a manner, the unevenness on the image surface relates to the glossiness.
Since there are various sizes of the recording member, many scratches due to the passage of paper edges exist on the surface of the fusing roller along the generating line direction. Hence, when fusing images on high glossy coated paper requiring high image quality, streaks causing low glossiness may be transferred or the glossiness difference due to the partial difference in surface roughness on the fusing roller may be generated.
There are methods for rendering scratches invisible on fixed images by overlapping invisible fine scratches over the scratches generated on the surface of the fusing roller and by finely skiving the surface layer of the fusing roller to expose new layer. However, granted that it is finely, the blemishing the surface reduces the surface nature, so that the reduced glossiness becomes a problem. Furthermore, when using a sliding member for overlapping invisible fine scratches or for finely skiving the surface layer, dust and foreign materials may be pinched, so that a secondary problem of vertical streaks generated in the circumferential direction has arisen.